Nitride semiconductor light-emitting device and method for fabrication thereof

ABSTRACT

A nitride semiconductor laser device uses a substrate with low defect density, contains reduced strains inside a nitride semiconductor film, and thus offers a satisfactorily long useful life. On a GaN substrate ( 10 ) with a defect density as low as 10 6  cm −2  or less, a stripe-shaped depressed portion ( 16 ) is formed by etching. On this substrate ( 10 ), a nitride semiconductor film ( 11 ) is grown, and a laser stripe ( 12 ) is formed off the area right above the depressed portion ( 16 ). With this structure, the laser stripe ( 12 ) is free from strains, and the semiconductor laser device offers a long useful life. Moreover, the nitride semiconductor film ( 11 ) develops reduced cracks, resulting in a greatly increased yield rate.

This nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Applications Nos. 2003-204262 and 2004-183163 filed in Japanon Jul. 31, 2003 and Jun. 22, 2004, respectively, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor light-emittingdevice, and to a method for fabricating one. More particularly, thepresent invention relates to a nitride semiconductor light-emittingdevice that uses a nitride semiconductor as a substrate thereof

2. Description of Related Art

There have been fabricated prototypes of semiconductor laser devicesthat oscillate in a region ranging from ultraviolet to visible light bythe use of a nitride semiconductor material as exemplified by GaN, AlN,InN, and composite crystals thereof. For such purposes, GaN substratesare typically used, and therefore GaN substrates have been intensivelyresearched by a host of research-and-development institutions. At themoment, however, no semiconductor laser devices offer satisfactorilylong useful lives, and accordingly what is most expected in them islonger useful lives. It is known that the useful life of a semiconductorlaser device strongly depends on the density of defects (such asvacancies, interstitial atoms, and dislocations, all disturbing theregularity of a crystal) that are present in a GaN substrate from thebeginning. The problem here is that substrates with low defect density,however effective they may be believed to be in achieving longer usefullives, are extremely difficult to obtain, and therefore researches havebeen eagerly done to achieve as much reduction in defect density aspossible.

For example, Applied Physics Letter, Vol. 73 No. 6 (1998), pp. 832-834,reports fabricating a GaN substrate by the following procedure. First,on a sapphire substrate, a 2.0 μm thick primer GaN layer is grown byMOCVD (metalorganic chemical vapor deposition). Then, on top of this, a0.1 μm thick SiO₂ mask pattern having regular stripe-shaped openings isformed. Then, further on top, a 20 μm thick GaN layer is formed again byMOCVD. Now, a wafer is obtained. This technology is called ELOG(epitaxially lateral overgrown), which exploits lateral growth to reducedefects.

Further on top, a 200 μm thick GaN layer is formed by HVPE (hydridevapor phase epitaxy), and then the sapphire substrate serving as aprimer layer is removed. In this way, a 150 μm thick GaN substrate isproduced. Next, the surface of the obtained GaN substrate is ground tobe flat. The thus obtained substrate is known to have a defect densityas low as 10⁶ cm⁻² or less.

It has been found, however, that, even with a semiconductor laser devicefabricated by growing, by a growing process such as MOCVD, a nitridesemiconductor film on a low-defect substrate, such as the one obtainedby the above-described procedure, it is still impossible to obtain auseful life that is satisfactorily long for practical use. Through anintensive study in search of the reason for that, the inventors of thepresent invention have found out that strains and cracks present insidea nitride semiconductor film greatly affect the deterioration and yieldrate of a semiconductor laser device. Even when a GaN substrate that ishomoepitaxial with a nitride semiconductor film is used, the grownnitride semiconductor film includes layers of InGaN, AlGaN, and the likewhose lattice constants and thermal expansion coefficients differ fromthose of GaN. The presence of these layers different from GaN causes anInGaN active layer and other layers to receive compressive stress. Ithas been found that the resulting strains present inside the filmaccelerate the deterioration of the semiconductor laser device.

Another problem with a nitride semiconductor film is many cracks thatdevelop therein, resulting in a low yield rate. The development of suchcracks is also greatly affected by strains present inside the film.

More specifically, when a laser structure formed of a nitridesemiconductor thin film is epitaxially grown on a nitride semiconductorsubstrate, many cracks (for example, several or more within a width of 1mm) develop, resulting in an extremely low yield rate of devices withthe desired characteristics. If a fabricated device contains cracks, thedevice may be flatly unable to produce laser oscillation at all, or,even if it can, its useful life is extremely short, making the devicepractically unusable. The development of such cracks is remarkable in adevice structure including a layer containing Al, and, since a nitridesemiconductor laser device typically includes such a layer, it is veryimportant to eradicate cracks.

SUMMARY OF THE INVENTION

In view of the conventionally encountered problems mentioned above, itis an object of the present invention to provide a nitride semiconductorlight-emitting device, such as a nitride semiconductor laser device,that uses a substrate with low defect density, that contains reducedstrains inside a nitride semiconductor film, and that thus offers asatisfactorily long useful life. It is another object of the presentinvention to provide a method for fabricating such a nitridesemiconductor light-emitting device with a high yield rate.

To achieve the above objects, in one aspect of the present invention, ina nitride semiconductor laser device including a substrate of which atleast a surface is a nitride semiconductor and a nitride semiconductorfilm laid on top of the surface of the substrate and having astripe-shaped laser light waveguide structure, the surface of thesubstrate has a low-defect region with a defect density of 10⁶ cm⁻² orless and a depressed portion, and the laser light waveguide structure ofthe nitride semiconductor film is located above the low-defect regionand off the depressed portion of the surface of the substrate.

In this nitride semiconductor laser device, a substrate having adepressed portion formed in a surface thereof is used, and a laser lightwaveguide structure formed in a nitride semiconductor film is locatedoff the area right above the depressed portion of the substrate. Whenthe nitride semiconductor film is grown, in the depressed portion of thesubstrate, growth progresses from different directions and a defectdevelops where growth from different directions meets, but, elsewhere,growth progresses regularly, suppressing meeting of growth accompaniedby defects. In the area above the low-defect region and off thedepressed portion, there are less defects resulting from defects of thesubstrate, and new defects are also less likely to develop, makingpresence of strains unlikely. By locating the laser light waveguidestructure of the nitride semiconductor film in this strain-free area, itis possible to give the device a long useful life. Moreover, even ifcracks develop, their development is limited within an area away fromthe laser light waveguide structure. This helps achieve an increasedyield rate.

Advisably, the depressed portion of the surface of the substrate isstripe-shaped. This makes it possible to suppress, over a wide area,meeting of growth accompanied by defects. Thus, it is possible to locatethe stripe-shaped laser light waveguide structure surely in anstrain-free area.

Advisably, the depressed portion of the surface of the substrate isgiven a horizontal cross-sectional area of 30 μm² or more. This helpsfurther reduce strains.

Alternatively, the depressed portion of the surface of the substrate isgiven a horizontal cross-sectional area of from 5 μm² to 30 μm², bothends inclusive, and the nitride semiconductor film is given a thicknessof from 2 μm to 10 μm, both ends inclusive. The smaller the depressedportion of the substrate is, and the thicker the nitride semiconductorfilm is, the less effectively the depressed portion reduces strains inthe nitride semiconductor film. With the cross-sectional area of thedepressed portion set within the range defined above, and in additionwith the thickness of nitride semiconductor film set within the rangedefined above, it is possible to satisfactorily reduce strains.

Alternatively, the surface of the substrate has, as the depressedportion, a plurality of depressed portions arranged at intervals of from50 μm to 2 mm, both ends inclusive. How effectively the depressedportion reduces strains in the nitride semiconductor film depends on thedistance from the depressed portion. With the interval at which theplurality of depressed portions are arranged set within the rangedefined above, it is possible to locate the laser light waveguidestructure surely in an strain-free area.

Preferably, the center of the laser light waveguide structure of thenitride semiconductor film is located 5 μm or more away from an edge ofthe depressed portion of the surface of the substrate. In an area withinthe nitride semiconductor film close to the area thereof located abovethe depressed portion, strains may develop under the influence ofmeeting of growth accompanied by defects. With the laser light waveguidestructure located so far away as defined above, it is possible to locatethe laser light waveguide structure surely in an strain-free area.

To achieve the above objects, in another aspect of the presentinvention, a method for fabricating a nitride semiconductor laser deviceincluding a substrate of which at least a surface is a nitridesemiconductor and a nitride semiconductor film laid on top of thesurface of the substrate and having a stripe-shaped laser lightwaveguide structure includes the steps of: forming a depressed portionon the substrate, which includes on the surface thereof a low-defectregion with a defect density of 10⁶ cm⁻² or less and; and locating thelaser light waveguide structure of the nitride semiconductor film abovethe low-defect region and off the depressed portion of the surface ofthe substrate.

Here, the substrate having the low-defect region and the depressedportion may be produced by forming, on a first nitride semiconductorhaving a low-defect region, a layer of a second nitride semiconductorand then removing at least part of the second nitride semiconductor.

To achieve the above objects, in still another aspect of the presentinvention, in a nitride semiconductor laser device including a substrateof which at least a surface is a nitride semiconductor and a nitridesemiconductor film laid on top of the surface of the substrate andhaving a stripe-shaped laser light waveguide structure, the surface ofthe substrate has a depressed portion, and the laser light waveguidestructure of the nitride semiconductor film is located above a regionlocated off the depressed portion of the surface of the substrate.

Alternatively, in a nitride semiconductor light-emitting deviceincluding a substrate of which at least a surface is a nitridesemiconductor and a nitride semiconductor film laid on top of thesurface of the substrate and having a light-emitting region, the surfaceof the substrate has a depressed portion, and the light-emitting regionof the nitride semiconductor film is located above a region located offthe depressed portion of the surface of the substrate.

Here, the depressed portion of the surface of the substrate may bestripe-shaped and formed in a mesh-like pattern.

To achieve the above objects, in a further aspect of the presentinvention, a method for fabricating a nitride semiconductorlight-emitting device including a substrate of which at least a surfaceis a nitride semiconductor and a nitride semiconductor film laid on topof the surface of the substrate and having a light-emitting regionincludes the steps of: forming, on the surface of the substrate, adepressed portion; forming, on the surface of the substrate having thedepressed portion formed therein, the nitride semiconductor film; andlocating the light-emitting region above a region located off thedepressed portion of the surface of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a sectional view and a top view, respectively,schematically showing the structure of a nitride semiconductor laserdevice embodying the invention;

FIGS. 2A and 2B are a top view and a sectional view, respectively,schematically showing the structure of the GaN substrate used in thenitride semiconductor laser device;

FIG. 3 is a top view schematically showing the wafer having a nitridesemiconductor grown on a conventional GaN substrate;

FIG. 4 is a sectional view schematically showing the layer structure ofthe nitride semiconductor film of the nitride semiconductor laserdevice;

FIG. 5 is a top view schematically showing the wafer having a nitridesemiconductor grown on the GaN substrate used in the nitridesemiconductor laser device;

FIG. 6 is a diagram showing the correlation between the depressioncross-sectional area, the nitride semiconductor film thickness, and theuseful life test yield rate;

FIGS. 7A and 7B are a sectional view and a top view, respectively,schematically showing the nitride semiconductor laser device with thenitride semiconductor film thereof made thicker;

FIGS. 8A and 8B are a top view and a sectional view, respectively,schematically showing another structure of the GaN substrate used in thenitride semiconductor laser device;

FIGS. 9A to 9C are sectional views each schematically showing adifferent pattern of the carved regions formed in the GaN substrate;

FIG. 10 is a top view showing the surface morphology of the GaNsubstrate;

FIG. 11 is a diagram showing height variations on the surface of thenitride semiconductor;

FIG. 12 is a diagram showing height variations on the surface of thenitride semiconductor; and

FIGS. 13A and 13B are a top view and a sectional view, respectively,schematically showing the structure of LEDs formed on the carvedsubstrate.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. In the following description, anegative index in a formula indicating a plane or direction of a crystalwill be represented by a negative sign “−” followed by the absolutevalue of the index instead of the absolute value accompanied by anoverscore placed thereabove as required by convention incrystallography, simply because the latter notation cannot be adopted inthe present specification.

EXAMPLE OF HOW A SUBSTRATE IS PRODUCED

Part of the fabrication process of a low-defect GaN substrate (with adefect density of 10⁶ cm⁻² or less) used in this embodiment can beachieved by the procedure described earlier in connection with theconventional example. Specifically, first, on a sapphire substrate, a2.5 μm thick primer GaN layer is grown by MOCVD. Then, on top of this, aSiO₂ mask pattern having regular stripe-shaped openings (with a periodof 20 μm) is formed. Then, further on top, a 15 μm thick GaN layer isformed again by MOCVD. Now, a wafer is obtained.

The film does not grow on SiO₂, and thus starts to grow inside theopenings. As soon as the film becomes thicker than the SiO₂, the filmthen starts to grow horizontally away from the openings. At the centerof every SiO₂ segment, different portions of the film growing fromopposite sides meet, producing, where they meet, a defect-concentratedregion with high defect density. Since the SiO₂ is formed in the shapeof lines, defect-concentrated regions are also formed in the shape oflines. As described earlier, this technology is called ELOG, whichexploits lateral growth to reduce defects.

Further on top, a 600 μm thick GaN layer is formed by HVPE (hydridevapor phase epitaxy), and then the sapphire substrate serving as aprimer layer is removed. Next, the surface of the obtained GaN substrateis ground to be flat. In this way, a 400 μm thick GaN substrate isproduced.

The substrate thus obtained has the GaN layer grown extremely thick, andthus includes, over almost the entire area thereof, low-defect regionswith a defect density of 10⁵ cm⁻² or less. However, depending on thegrowth conditions, regions with a defect density higher than 10⁵ cm⁻²may be formed in the shape of stripes on the surface of the obtainedsubstrate in such a way as to correspond to the defect-concentratedregions mentioned above. It should be noted that, also during growth byHVPE, by forming a SiO₂ mask above the defect-concentrated regions, itis possible to more effectively reduce defects on the surface of thesubstrate.

Here, the substrate is produced by ELOG. It should be understood,however, that how the substrate is produced does not affect in any waythe nature and effects of the present invention. Specifically, the onlyrequirement is to use a nitride semiconductor substrate having alow-defect region on the surface thereof.

The dislocation density can be evaluated by one of the following andother methods. One way is by subjecting the substrate to etching bydipping it in a mixed acid liquid, namely a mixture of sulfuric acid andphosphoric acid, heated to 250° C. and then counting the EPD (etch pitdensity) within a 50 μm×50 μm region. Another way is by counting thedislocation density in an image obtained by using a transmissionelectron microscope.

The measurement of the EPD can be made possible by the use of gas-phaseetching such as RIE. Alternatively, suspension of growth in a MOCVDfurnace followed by exposure to a high temperature (about 1,000° C.)also makes the measurement of the EPD possible. The measurement itselfcan be achieved by the use of an AFM (atomic force microscope), CL(cathode luminescence), microscopic PL (photo luminescence), or thelike.

It should be noted that a substrate with low defect density denotes notonly a substrate having low-defect regions distributed all over the areathereof but also a substrate including low-defect regions in only aportion of the surface thereof. The low-defect regions may bedistributed in any manner, but the laser stripes of semiconductor lasersneed to be so formed as to include those low-defect regions.

Forming a Semiconductor Laser Device

By the procedure described earlier or the like, a GaN substrateincluding low-defect regions is obtained. In this embodiment, thesubstrate is assumed to have a defect density of about 10⁶ cm⁻² or lessover the entire area thereof. Next, all over the surface of thissubstrate, SiO₂ or the like is vapor-deposited so as to have a thicknessof 1 μm by sputtering. Then, by common photolithography, stripe-shapedwindows are formed with photoresist so as to have a width of 40 μm eachand a period of 400 μm in the [1-100] direction. Then, by ICP or RIE(reactive ion etching), the SiO₂ and the GaN substrate are etched. TheGaN substrate is etched to a depth of 6 μm. Thereafter, the SiO₂ isremoved with an etchant such as HF. This is the end of the treatment ofthe substrate to be performed before a nitride semiconductor film isgrown thereon.

FIGS. 2A and 2B show the thus obtained substrate, in a top view and asectional view, respectively. Reference numeral 21 represents the GaNsubstrate, and reference numeral 22 represents the regions etched byRIE. Symbols X, Z, and T represent the etching width, depth, and period,respectively. The etching may be achieved by the use of gas-phaseetching, or by the use of a liquid etchant. In the followingdescriptions, the regions 22 of the substrate depressed as a result ofbeing removed by etching will be referred to also as the carved regions.The carved regions may be formed after once the thin film of GaN, InGaN,AlGaN, InAlGaN, and the like has been grown on the GaN substrateincluding low-defect regions. That is, the present invention includesstructures wherein a nitride semiconductor film is grown by firstgrowing it and then forming carved regions.

The carved regions may be arranged in one of various patterns. Forexample, as shown in FIGS. 8A and 8B, two carved regions may be formedat a predetermined interval; or, as shown in FIGS. 9A to 9C, more thantwo carved regions may be formed, or carved regions may be formed withdifferent periods used mixedly, or carved regions may be formed in amixed pattern including singly and doubly arranged carved regions. Thepresent invention is applicable as it is so long as the cross-sectionalarea and period of one carved region is within the ranges defined in theappended claims. In cases where different periods are used mixedly, eachof those mixedly used periods needs to be within the range defined inthe appended claims. Here, reference numerals 81 and 91 represent theGaN substrate, and reference numerals 82 and 92 represent the carvedregions.

In FIGS. 1A, 1B, 2A, 2B, 5, 7A, 7B, 8A, and 8B, the carved regions areformed parallel to the [1-100] direction. It is, however, also possibleto form them in another direction, for example parallel to the [11-20]direction. Basically, the effects of the present invention do not dependon the direction of carving, and therefore the carved regions may beformed in any direction.

The substrate used may include a region with high defect density. This,however, may lead to degraded surface morphology during epitaxialgrowth, and therefore it is preferable to use a substrate including noregion with high defect density.

On this substrate, the nitride semiconductor film shown in FIG. 4 isgrown, and thereby the nitride semiconductor laser device of thisembodiment is obtained. FIGS. 1A and 1B schematically show the structureof the thus obtained semiconductor laser device. FIG. 1A is a sectionalview of the semiconductor laser device as seen from the direction inwhich it emits light, and FIG. 1B is a top view of the semiconductorlaser device as seen from above.

Here, reference numeral 10 represents the GaN substrate, and, in thissubstrate 10, low-defect regions are present. FIG. 4 shows thefollowing. On an n-type GaN layer (1.0 μm) 40, there are laid thefollowing layers one on top of another in the order mentioned: an n-typeAl_(0.062)Ga_(0.938)N first clad layer (1.5 μm) 41, an n-typeAl_(0.1)Ga_(0.9)N second clad layer (0.2 μm) 42, an n-typeAl_(0.062)Ga_(0.938)N third clad layer (0.1 μm) 43, an n-type GaN guidelayer (0.1 μm) 44, an In GaN/GaN-3MQW active layer (InGaN/GaN=4 nm/8 nm)45, a p-type Al_(0.3)Ga_(0.7)N evaporation prevention layer (20 nm) 46,a p-type GaN guide layer (0.05 μm) 47, a p-type Al_(0.062)Ga_(0.938)Nclad layer (0.5 μm) 48, and a p-type GaN contact layer (0.1 μm) 49.

On top of the substrate 10, a nitride semiconductor film (epitaxiallygrown layer) 11 is formed that has the same structure as the nitridesemiconductor thin film shown in FIG. 4. Moreover, on the top surface ofthe nitride semiconductor film 11, a laser stripe 12 is formed as alaser light waveguide structure. This laser stripe 12 needs to be formedso as to be located above a low-defect region included in the substrate.The substrate used in this embodiment has low-defect regions all overthe area thereof, and therefore the laser stripe may be formed anywherethereon except above carved regions. The reason will be described later.

On the top surface of the nitride semiconductor film 11 is formed SiO₂13 for current narrowing, and on the top surface of this is formed ap-type electrode 14. On the other hand, on the bottom surface of thesubstrate 10 is formed an n-type electrode 15. Reference numeral 16represents a carved region. The top surface of the portion of thenitride semiconductor film 11 located above the carved region 16 isdepressed under the influence of the carved region 16.

Whether the top surface of the portion located above the carved region16 is depressed or not depends on the thickness of the nitridesemiconductor film. FIG. 7 schematically shows the structure obtainedwhen the top surface of the portion located above the carved region isflat. In FIG. 7, reference numeral 70 represents the GaN substrate,reference numeral 71 represents the nitride semiconductor film,reference numeral 72 represents the laser stripe, reference numeral 73represents SiO₂ for current narrowing, reference numeral 74 representsthe p-type electrode, reference numeral 75 represents the n-typeelectrode, and reference numeral 76 represents the carved region. Inthis way, as the nitride semiconductor film is made thicker, the topsurface of the portion located above the carved region 76 becomesflatter. It should be noted that, in the present invention, whether thetop surface of the portion located above the carved region is depressedor flat does not matter.

In FIG. 1A, the distance from the center of the laser stripe 12 to theedge of the carved region 16 is represented by “d”, and specificallyd=40 μm here. FIG. 5 schematically shows a top view of the wafer beforeit is diced into individual semiconductor laser devices. In thisembodiment, it was possible to obtain a nitride semiconductor film 51completely free from cracks all over the area thereof. In FIG. 5,reference numeral 52 represents the carved regions.

The wafer can be diced into individual nitride semiconductor laserdevices by a common dicing process. No description will be given of thisdicing process. No cracks were observed in the nitride semiconductorlaser devices after the separation of the wafer into individual chips.As a result, the laser devices oscillated with stable characteristics,and the yield rate of the semiconductor laser devices of this embodimentwhich offered the desired oscillation characteristics (i.e., thoserequiring a drive current lop of 70 mA or less when producing an opticaloutput of 30 mW) was more than 90%.

The useful lives of the diced semiconductor laser devices were testedwith the devices driven under APC (automatic power control) at 60° C.and at an output of 30 mW. In the test, the devices emitted atwavelengths of 405±5 nm. From each wafer, 50 semiconductor laser devicesthat fulfilled predetermined initial characteristics were randomlypicked out, and the number of devices of which the useful lives exceeded3,000 hours was counted as the yield rate. Here, the yield rate of thesemiconductor laser device of this embodiment was more than 85%.

COMPARATIVE EXAMPLE 1 OF A SEMICONDUCTOR LASER DEVICE

Now, with reference to FIG. 3, a description will be given of whathappened when, without any extra treatment performed, a nitridesemiconductor film was grown on a substrate including low-defect regionsin the surface thereof. Reference numeral 31 represents a wafer producedby growing, by MOCVD, a nitride semiconductor thin film as shown in FIG.4 on a substrate including low-defect-density regions. Reference numeral32 represents cracks that developed in the wafer.

When a nitride semiconductor film was grown on a substrate includinglow-defect regions as it is (i.e., if no carved regions were formedthereon), as shown in FIG. 3, many cracks developed in the wafer. Theresult of counting the number of cracks that crossed a 1 mm×1 mm regionon the wafer was about three to ten. If a fabricated device containscracks, the device may be flatly unable to produce laser oscillation atall, or, even if it can, its useful life is extremely short, making thedevice practically unusable. For this reason, the yield rate of devicesthat produced the desired laser oscillation was extremely low,specifically 50% or less. The development of such cracks is remarkablein a device structure including a layer containing Al, and, since anitride semiconductor laser device typically includes such a layer, itis very important to eradicate cracks.

Moreover, when semiconductor laser devices were fabricated in portionsof the wafer where no cracks happened to develop and their useful liveswere tested at 60° C. and at 30 mW, the yield rate of devices with auseful life of 3,000 hours or more was as poor as about 15%. One causefor this is considered to be minute cracks that are present in the waferbut that cannot be observed from the surface of the wafer. Here, theuseful life is defined as the length of time required for the drivecurrent Iop to become 1.5 times the initial level thereof while theoutput is kept at 30 mW.

The embodiment under discussion aims to obtain a long useful life byreducing cracks, increasing the yield rate of semiconductor laserdevices, and controlling the strains that develop therein. Now, theembodiment will be described in detail.

COMPARATIVE EXAMPLE 2 OF A SEMICONDUCTOR LASER DEVICE

Another comparative example of a semiconductor laser device wasfabricated in which a laser stripe was formed above a carved region.Except for the position of the laser stripe, this semiconductor laserdevice has the same structure as that of the embodiment underdiscussion. The useful lives of semiconductor laser devices diced out ofa wafer having a structure wherein laser stripes were formed right abovecarved regions 16. In this test, the yield rate was 35% or less. Thelessening of the yield rate here is believed to result from severerstrains contained in the semiconductor laser devices. The fact that,when no carved regions were formed in the substrate, many cracksdeveloped is considered to suggest the presence of considerably severestrains.

In the portion above a carved region, the film grows horizontally fromthe noncarved portions contiguous therewith, and thus the film runs intothe carved portion. At this time, pressure acts from both sides on theportion above the carved region, and this is considered to cause thisportion to contain severer strains than noncarved regions. Moreover, thecarved region has walls at both sides, and therefore the growth thattends to advance to both sides is hampered by the walls. This alsoresults in contained strains. Growth in the carved region iscomplicated; specifically, growth advances from different directions(normal growth that advances from the bottom surface of the carvedregion, growth that advances from the side faces of the carved region,growth resulting from running-in from noncarved regions, etc.). Thus,not only does the severity of strains vary within a region, thedirection of strains also differs from one place to another, resultingin poor repeatability and thus instability. This is considered to be thecause of the lessening of the yield rate.

Moreover, since growth advances from different directions, manydislocations, defects, and the like develop where growth from differentdirections meets. Accordingly, when laser stripes are formed abovecarved regions, such dislocations, defects, and the like promotedeterioration, making it impossible to obtain long useful lives.

On the other hand, a noncarved region, when growing, runs into carvedregions, and can thereby release strains. This releasing of strainssuppresses development of cracks, and simultaneously releases thestrains contained in the noncarved region. This releasing of strainsoccurs with good repeatability and with stability. Moreover, as opposedto the portion above a carved region, growth does not advance fromdifferent directions. This helps produce a satisfactorily crystallinefilm free from dislocations, defects, and the like. These are believedto be the reasons that forming laser stripes in noncarved regionsincreases the reliability of semiconductor laser devices and prolong theuseful lives thereof.

In this embodiment, by forming carved regions 16 and forming laserstripes 12 elsewhere than right above the carved regions 16, it ispossible to greatly increase the reliability of LD devices, to suppressthe development of cracks, and to dramatically improve the yield rate.

Study on the Carving Conditions and the Layer Thickness

Moreover, the inventors of the present invention have found out that theyield rate correlates with the carving width X (see FIG. 2B) of thecarved regions, the carving depth Z thereof, and the total filmthickness of the nitride semiconductor film grown on the substrate. Thetotal film thickness of the nitride semiconductor film is the totalthickness of all the layers thereof including from the n-type GaN layer40 through the p-type GaN contact layer 49 shown in FIG. 4.

Here, the total film thickness was adjusted so as to be varied from 2 μmto 30 μm. FIG. 6 shows the results of plotting the yield rate measuredby the useful life test described above against different combinationsof the carving cross-sectional area, i.e., the carving depth Z (in μm)multiplied by the carving width X (in μm), and the total film thicknessof the nitride semiconductor film grown on top thereof. The period T(see FIG. 2B) of the carved regions was 400 μm.

The regions in which the yield rate was found to be high by the usefullife test indicate that, there, the development of cracks waseffectively suppressed and the strains in noncarved regions (whereridges are formed) were effectively released. With the carving widthX=10 μm and the carving depth Z=5 μm, the carving cross-sectional areaequals 5×10=50 μm². The carving width X was varied in the range from the3 μm to 200 μm, and the carving depth Z was varied in the range from 0.5μm to 30 μm.

As shown in FIG. 6, when the carving cross-sectional area was 30 μm² ormore, it was possible to obtain high yield rates irrespective of thetotal film thickness of the nitride semiconductor film grown on thesubstrate. This is considered to be because the strains in noncarvedregions were effectively and stably released. Although FIG. 6 onlycovers up to a cross-sectional area of 100 μm², it was in fact possibleto obtain yield rates of 80% or more up to a cross-sectional area of2,000 μm² in the above film thickness range.

When the carving cross-sectional area was in the range from 5 μm² to 30μm², so long as the total film thickness of the nitride semiconductorfilm grown on the substrate was 10 μm or less, it was possible to obtainimproved yield rates. When the carving cross-sectional area was lessthan 5 μm², no improvement was obtained if the total film thickness ofthe nitride semiconductor film was in the range from 2 μm to 30 μm. Thisis considered to be because the carving cross-sectional area was sosmall that the strains in noncarved regions were not releasedeffectively.

FIG. 6 shows the results obtained when the period T of the carvedregions was 400 μm. Similar tests were conducted with varying periods T.The period T was varied in the range from 50 μm to 2 mm. The carvingwidth X was varied in the range up to one-half of the period T. Forexample, when the period T was 50 μm, the carving width X was varied inthe range from 0 to 25 μm.

When the period T was in the range from 50 μm to 2 mm, a tendencyapproximately identical with that shown in FIG. 6 was observed.Specifically, as shown in FIG. 6, when the carving cross-sectional areawas 30 μm² or more, it was possible to obtain high yield ratesirrespective of the total film thickness of the nitride semiconductorfilm grown on the substrate; when the carving cross-sectional area wasin the range from 5 μm² to 30 μm², so long as the total film thicknessof the nitride semiconductor film grown on the substrate was 10 μm orless, it was possible to obtain improved yield rates; when the carvingcross-sectional area was less than 5 μm², no improvement was obtained ifthe total film thickness of the nitride semiconductor film was in therange from 2 μm to 30 μm.

Study on the Position of the Stripe

With respect to where to form ridges, forming them at a distance of 5 μmor less from the edges of carved regions 16 resulted in greatlyshortened useful lives in the useful life test. This is considered to bebecause severe strains were present around carved regions. Accordingly,laser stripes need to be formed 5 μm or more away from the edges ofcarved regions. Moreover, the position of laser stripes needs to be sodetermined that they are formed not only in regions with mild strainsbut in regions with high flatness.

A substrate including carved regions suffers from variations in thethickness of the epitaxially grown layer which are observed around thecarved regions. FIG. 10 is a diagram illustrating this situation, andshows the state of a wafer 1001 produced by epitaxially growing, byMOCVD, a plurality of nitride semiconductor layers (for example, with atotal film thickness of 5 μm) on a GaN substrate having carved regions1002 formed thereon so as to be parallel to one another. In the regionsbetween the grooves, there are formed semiconductor laser waveguidestripes 1004 (of which the position is indicated by broken lines).

It is inevitable that, under the influence of the grooves, the filmthickness of the grown layer varies according to the distance from thegrooves. In reality, however, evaluating the layer thickness even at afixed distance from a groove along the direction of the grooves revealsvariations in the layer thickness. Moreover, when the surface of thewafer is inspected under an optical microscope, wave-like morphology1005 is observed as schematically shown in the figure.

This is considered to be because how the crystal grows in the regions1003 between the carved regions is sensitively influenced by thegrooves. If laser waveguide stripes are formed in such regions,variations in the film thickness along the wave guides not onlyadversely affect the laser characteristics but also make thecharacteristics of individual devices uneven.

By contrast, in regions 30 μm or more away from the edges of the carvedregions, the above-described variations in the film thickness of thegrown layer rapidly diminish, making wave-like surface morphology asshown in FIG. 10 unobservable.

Along the direction of arrow X shown in FIG. 10, height variations onthe surface were measured by using a height difference measurementmachine. The measurements were made by using the “DEKTAK3ST” modelmanufactured by A SUBSIDIARY OF VEECO INSTRUMENTS INC. The measurementconditions were: measurement length, 2,000 μm; measurement duration, 3minutes; probe needle pressure, 30 mg; horizontal resolution, 1 μm persample. FIG. 11 shows the results of measuring height variations innoncarved regions 30 μm away from carved regions. FIG. 12 shows theresults of measuring height variations in noncarved regions 5 μm awayfrom carved regions. As will be understood from FIGS. 11 and 12, whereasheight variations on the surface in regions 30 μm away from carvedregions were about 40 nm, those in regions 5 μm away therefrom were aslarge as 200 nm.

In a semiconductor laser, the laser stripe needs to be formed at acertain distance, 5 μm or more at least (preferably, 30 μm or more), tosuppress variations (strains and flatness) resulting from the influenceof grooves as described in connection with the related art. In such aposition, lateral growth does not effectively suppress the propagationof defects from the substrate.

The purpose of forming grooves in the substrate according to the presentinvention is utterly different from the purpose of forming grooves in asubstrate with a view to exploiting so-called lateral growth technology(for example, ELOG technology) to reduce the density of defectsextending from the substrate to a crystal growth film. For the purposeof reducing the defect density, to obtain the effect of lateral growth,the intervals between the grooves are typically about equal to the filmthickness of the formed layer or less, and are, even extended to themaximum, about three times the film thickness or less. In thisstructure, it is difficult to obtain regions where, as described above,the layer thickness is uniform in the direction parallel to the grooves.Thus, when laser stripes are formed, undesirably, the film thicknessvaries in the direction of the stripes.

By contrast, the grooves in the present invention are formed not forthat purpose, but for the purpose of maintaining a certain degree offlatness where the laser stripe is formed while effectively preventingcracks. The intervals of the grooves are about of the same order as thewidth of the semiconductor laser device, specifically about 50 μm at theminimum, and preferably 100 μm or more.

Here, the description deals specifically with a semiconductor laser. Itshould be understood, however, that application of the present inventionis not limited to this particular type of device. For example, even in acase where an electronic device such as a light-emitting diode (LED) orFET (field-emission transistor) is formed on a substrate as described inconnection with this embodiment, on the same principles as thosedescribed above, it is possible to greatly reduce strains and crackspresent in a nitride semiconductor film and thereby increase the yieldrate. With an LED or the like, problems have been reported such as anuneven light emission pattern and lowering of light emission intensitycaused by strains present in a film.

In such a device, carved regions 131 may be formed in the shape ofstripes that run both longitudinally and laterally so as to form amesh-like pattern as shown in FIGS. 13A and 13B. In FIGS. 13A and 13B,reference numeral 132 represents an n-type GaN substrate, referencenumeral 133 represents a p-type electrode, reference numeral 134represents an n-type electrode, reference numeral 135 represents anitride semiconductor thin film. When an LED was fabricated with thestructure shown in FIGS. 13A and 13B, it was possible to reduce thestrains present in the nitride semiconductor film, to alleviate theunevenness in the light emission pattern, and reduce cracks to zero.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced other than as specifically described.

1. A nitride semiconductor laser device comprising: a substrate of whichat least a surface is a nitride semiconductor; and a nitridesemiconductor film laid on top of the surface of the substrate andhaving a stripe-shaped laser light waveguide structure, wherein thesurface of the substrate has a low-defect region with a defect densityof 10⁶ cm⁻² or less and a depressed portion, and wherein the laser lightwaveguide structure of the nitride semiconductor film is located abovethe low-defect region and off the depressed portion of the surface ofthe substrate.
 2. The nitride semiconductor laser device of claim 1,wherein the depressed portion of the surface of the substrate isstripe-shaped.
 3. The nitride semiconductor laser device of claim 2,wherein the depressed portion of the surface of the substrate has across-sectional area of 30 μm² or more.
 4. The nitride semiconductorlaser device of claim 2, wherein the depressed portion of the surface ofthe substrate has a cross-sectional area of from 5 μm² to 30 μm², bothends inclusive, and the nitride semiconductor film has a thickness offrom 2 μm to 10 μm, both ends inclusive.
 5. The nitride semiconductorlaser device of claim 2, wherein the surface of the substrate has, asthe depressed portion, a plurality of depressed portions arranged atintervals of from 50 μm to 2 mm, both ends inclusive.
 6. The nitridesemiconductor laser device of claim 2, wherein a center of the laserlight waveguide structure of the nitride semiconductor film is located 5μm or more away from an edge of the depressed portion of the surface ofthe substrate.
 7. A method for fabricating a nitride semiconductor laserdevice including a substrate of which at least a surface is a nitridesemiconductor and a nitride semiconductor film laid on top of thesurface of the substrate and having a stripe-shaped laser lightwaveguide structure, the method comprising the steps of: forming adepressed portion on the substrate, which includes on the surfacethereof a low-defect region with a defect density of 10⁶ cm⁻² or less;and locating the laser light waveguide structure of the nitridesemiconductor film above the low-defect region and off the depressedportion of the surface of the substrate.
 8. The method of claim 7,wherein the substrate having the low-defect region and the depressedportion is produced by forming, on a first nitride semiconductor havinga low-defect region, a layer of a second nitride semiconductor and thenremoving at least part of the second nitride semiconductor.
 9. A nitridesemiconductor laser device comprising: a substrate of which at least asurface is a nitride semiconductor; and a nitride semiconductor filmlaid on top of the surface of the substrate and having a stripe-shapedlaser light waveguide structure, wherein the surface of the substratehas a depressed portion, and wherein the laser light waveguide structureof the nitride semiconductor film is located above a region located offthe depressed portion of the surface of the substrate.
 10. A nitridesemiconductor light-emitting device comprising: a substrate of which atleast a surface is a nitride semiconductor; and a nitride semiconductorfilm laid on top of the surface of the substrate and having alight-emitting region, wherein the surface of the substrate has adepressed portion, and wherein the light-emitting region of the nitridesemiconductor film is located above a region located off the depressedportion of the surface of the substrate.
 11. The nitride semiconductorlight-emitting device of claim 10, wherein the depressed portion of thesurface of the substrate is stripe-shaped, and is formed in a mesh-likepattern.
 12. A method for fabricating a nitride semiconductorlight-emitting device including a substrate of which at least a surfaceis a nitride semiconductor and a nitride semiconductor film laid on topof the surface of the substrate and having a light-emitting region, themethod comprising the steps of: forming, on the surface of thesubstrate, a depressed portion; forming, on the surface of the substratehaving the depressed portion formed therein, the nitride semiconductorfilm; and locating the light-emitting region above a region located offthe depressed portion of the surface of the substrate.